1. Field of the Invention
The present invention relates to a pressure regulating unit for regulating pressure to be applied to liquid, a liquid supplying apparatus provided with the pressure regulating unit, and a liquid ejecting apparatus.
2. Description of the Related Art
A liquid ejecting apparatus is exemplified by an inkjet printing apparatus for ejecting ink (i.e., liquid) supplied from an ink tank (i.e., a liquid container), ink being ejected from an inkjet print head (i.e., a liquid ejection head) so as to print an image on a print medium. In, for example, a business-grade printing apparatus requiring an improved print speed as the printing apparatus, a large quantity of ink is ejected from the print head. In view of this, a large quantity of ink is required to be supplied to the print head. For this purpose, ink contained in the ink tank is pressurized by a pump or the like, and then, the pressurized ink need be supplied (pressure-supplied). In the meantime, ink, to which a negative pressure is applied, need be supplied to the print head for ejecting the ink from an ejection port in order to suppress the leakage of the ink from the ejection port.
Japanese Patent No. 3606282 discloses an apparatus for regulating the pressure of ink, the apparatus including a pressure regulating unit provided on a supply channel, through which the ink is supplied to a print head. The pressure regulating unit is adapted to regulate the pressure of ink to be pressure-supplied, and to apply a negative pressure to the ink, and then, to supply the ink to the print head. The negative pressure is applied to the ink to be supplied to the print head so as to form a meniscus of the ink at an ejection port. The pressure regulating unit opens or closes a valve disposed on the ink supply channel according to the negative pressure inside of the print head, the negative pressure inside of the print head being varied according to the ejection of the ink, in order to stabilize the negative pressure of the ink inside of the print head.
In a business-grade printing apparatus, in particular, the size of a print head has become greater in recent years so as to further increase a print speed, and therefore, an ink ejection frequency has become higher. Accordingly, an ink supply quantity with respect to the print head is increased.
As the ink supply quantity is increased, a pressure loss in the ink supply channel between the pressure regulating unit and the print head becomes large. The pressure loss causes a possibility that the ink inside of the print head cannot be kept at a proper negative pressure. For example, there is a possibility that the pressure loss causes a difference between a predetermined ink negative pressure regulated by the pressure regulating unit and the negative pressure of the ink inside of the print head, thereby making the negative pressure of the ink inside of the print head insufficient, resulting in ink ejection deficiency.